1. Field of the Invention
The invention relates to raw gas/purified gas heat exchanger, particularly for dust-loaded raw gases.
2. Description of the Prior Art
Heat exchangers are known in which hot gas, as the primary medium, heats another gas serving as a secondary medium. It is also known that the greatest possible heating of the secondary medium is achieved if the latter flows through the heat exchanger in counterflow to the primary medium. With heavily dust-laden gases such as are generated especially after powdered coal furnaces, fluidized-bed furnaces or coal gasifiers, the problem arises, however, to reduce or remove the deposits which have a detrimental effect on the heat transfer. This involves not always only deposits of soot, dust or ash particles but also, particularly behind coal gasifiers, the desublimation of NH.sub.4 Cl as well as deposits which are caused by thermophoretic effects. These deposits can clog heat exchanger tubes with internal flow or if the flow is on the outside, form deposits on the heat exchanger tubes which gradually narrow down the spaces between these heat exchanger tubes and finally also clog them. These deposits decrease the heat exchange distinctly if they have a thickness of one to two millimeters.
It has been proposed to let the dust-laden raw gases flow through the heat exchanger tubes at a high flow velocity which prevents the formation of deposits in the tubes. This solution, however, requires considerable compressor power and, if the raw gas is loaded with dust, leads to erosion problems at the heat exchanger tubes and the compressors.